Visible and near-infrared luminescence of helical zinc(II)-lanthanide(III) trinuclear complexes having acyclic Bis(N₂O₂) oxime ligand

Abstract

Luminescent properties of helical zinc(II)-lanthanide(III) complexes [LZn2Ln] (Ln = La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+), which are obtained by the reaction of acyclic bis(N2O2) tetraoxime ligand with zinc(II) and lanthanide(III) ions, were studied. The 1H NMR spectral study indicated that the complexes [LZn2Ln] adopt a helical conformation in solution, where two terminals are in an equivalent environment. In the absorption spectra, the π-π* transitions of ligand L4− were observed at around 330-340 nm associated with two shoulder bands between 370-450 nm. Upon excitation at 425 nm, [LZn2La] showed relatively intense ligand-centered emission at 540 nm (quantum yield, 0.052). The corresponding luminescence of the complexes of all other lanthanides [LZn2Ln] (Ln = Ce3+ -Lu3+) was much weaker. The complexes [LZn2Ln] (Ln = Sm3+, Eu3+, Tb3+, Dy3+) did not show detectable metal-centered emission, although these ions are potentially emissive in the visible region. On the other hand, [LZn2Nd] and [LZn2Yb] exhibited metal-centered emission due to f-f transition in the near infrared region in addition to the weak ligand-centered emission. Consequently, the ligand L4− can work as a sensitizer for Nd3+ and Yb3+ luminescence in the near infrared region.